1. Field of the Invention
This invention relates to the field of seismic data acquisition and processing. More particularly, this invention relates to a method of using 2-dimensional extrapolation techniques for predicting lines of seismic data beyond existing lines of seismic data and interpolating lines of seismic data between existing lines of seismic data.
2. Background of the Art
3-D marine seismic surveys entail towing a swath of elongated seismic sensor arrays. The swaths are repeated to increase the amount of coverage in a direction transverse to the direction of the sensor arrays. In order to reduce the cost of acquisition, it is desirable to avoid overlap of successive swaths and, if possible, not leave gaps between the successive swaths.
The present invention may be applied to any form of seismic operation, be it on land or on sea. However for convenience, by way of example but not by way of limitation, this disclosure will be explained in terms of a marine seismic survey.
In 3-D marine operations, a seismic ship tows a swath including a plurality of parallel seismic streamer cables along a desired line of survey, the cables being submerged by a few meters beneath the water surface. The number of cables that make up a swath depends only on the mechanical and operational capabilities of the towing ship. There may be six or more such cables, spaced about 50 to 100 meters apart. The respective cables may be up to 8 to 12 km. long.
Each streamer cable typically includes 120 or more spaced-apart seismic detector groups. Each group consists of one or more individual interconnected detectors, each of which services a single data channel. The group spacing is on the order of 25 to 50 meters longitudinally along the cable. The seismic detectors are transducers that perceive the mechanical activity due to reflected acoustic wavefields and convert that activity to electrical signals having characteristics representative of the intensity, timing and polarity of the acoustic activity as is well known to the art. The detectors are operatively coupled to data-storage and processing devices of any desired type.
An acoustic source such as an array of air guns, is towed in the water by the ship near the leading end of the swath of seismic streamer cables. As the ship proceeds along the line of survey, the source is fired (activated) at selected spatial intervals, commonly equal to the group interval. Assuming the ship travels at a constant velocity such as 4-6 knots, the source may be conveniently fired at selected time intervals such as every 5-13 seconds or more. The wavefield emitted by the source, travels downwardly to be reflected from subsea earth formations whence the wavefield is reflected back to the water surface where the reflected wavefield is received by the detectors and converted to electrical signals. The detected electrical signals are transmitted to any well-known signal recording and processing means for providing a physical model of the subsurface.
FIG. 1 is a plan view of a 3-D seismic survey acquisition. A first 3-D swath 13 of six parallel seismic cable arrays A1-A6 are towed through a body of water by a ship 14. The cables are typically configured to be a distance d apart. It should be understood that six cables are shown only as an example, and a greater or lesser number of cables could be used in the acquisition. It should be further understood that, if land operations are under consideration, the cables could be towed by one or more trucks or could be laid out by cable trucks using roll-along techniques in a manner well-known to the seismic industry. Signals from the respective cable arrays A1-A6 are fed over a data-signal manifold 20 to a processor 22 of any well-known type, installed on ship 14 and operatively coupled to the processor 22 by electrical lead-ins 16 and 18. A discrete acoustic source SL is towed by ship 14 near the leading end of swath 13, substantially at the center of the swath. More than one discrete source such as SL' and SL", offset from the center line may be used if desired.
A second 3-D swath 13' of six parallel seismic cable arrays A1'-A6' may be obtained by towing the cables at a position that is laterally offset from the first swath 13.
This may be done by a ship 14'. It should be understood that the second swath could be obtained at a later time than the first swath, so that the ship 14' could be the same as the ship 14, or it could be acquired simultaneously with the acquisition of the first swath 13 using a second ship. Without limitation, the invention is discussed herein as if the difference swaths are acquired at different times using the same ship.
Those versed in the art would recognize that in order to minimize the cost of seismic data acquisition, the distance D between the two swaths 13 and 13' should, at the very least, be the same as the distance d between the cables. If this is not done, there will be unnecessary duplication of data. Those versed in the art would also recognize that if the distance D is greater than the distance d, data acquisition costs could be reduced if the data within the gap between the swaths could be filled in by simulating data that would be acquired by cables within the gap. The term "gap" here is not limited to spatial gaps in the acquisition geometry and is particularly intended to include a gap in the illumination of the subsurface by the seismic acquisition. The term "insonification" has sometimes been used to describe illumination by sound waves. The quality of a processed image that is based partially on such simulated data depends upon how the simulation is done. Even if only one swath of data is acquired, it would be desirable to simulate additional cables of data that are simply extrapolated away from an existing swath of data. Another problem that is encountered in 3-D seismic data acquisition is that of aliasing, wherein due to inadequate sampling of data, certain frequencies and/or dips in the subsurface cannot be properly imaged. Interpolation of data between existing cables is one possible solution to the problem. The present invention addresses the need for such extrapolation and interpolation of cables of seismic data.